Air springs (sometimes called “air bags”) are commonly used in large vehicles (e.g. unloaded weight of over 5 tons) such as tractor-trailers, as springs to support the vehicle chassis on axles and to support the cab on the chassis. A vehicle manufacturer sets the preferred air spring heights, such as 15 inches ±0.25 inch, to maintain good suspension geometry, drive line angle (angle of the transmission vs. angle of the differential), etc. A leveling system such as one with a rotating shaft shown in U.S. Pat. No. 6,202,992, or one with a vertically sliding part shown in US publication 2008/0315539, includes a height sensor that controls a leveling valve. The leveling valve controls the flow of pressured air from a pressured air source into the air spring to increase its height and controls the outflow of air from the air spring to the atmosphere to decrease its height. With the leveling valve mounted on the chassis, the height sensor is typically in the form of a lever that extends between the axle and the chassis. Pivoting of the lever moves a valve member to operate the leveling valve. The leveling valve and height sensor are purely mechanical devices that operate without an electrical input or output.
A blocking valve may be placed between the air pressure source and the leveling valve, with the blocking valve controlled to stop the flow of air into or out of the air spring in situations that indicate that an air spring height change is temporary. An example of such temporary changes is when the air spring height repeatedly changes due to the vehicle travelling over a rough road that causes the vehicle to bounce up and down. It is desirable to minimize the flow of air from the pressured source because such flow results in an increased use of fuel. However, present leveling valves are already constructed to minimize pressured air flow, as by dampeners that normally allow only a very slow increase in rate of air flow into and out of the air spring. Any system for further reducing wastage of pressured air must be capable of being installed at low cost in order to gain acceptance.